Arrange to take a photo that is standard for the industry. Industry use for headshots is 8-by-10 inches. The picture can be either matte or glossy finish with a white border. Stay away from fancy fonts or colorful borders that distract from your face.
Take a picture that shows your full face and some of your shoulders. This is called a ¾ photo. The casting agent should be able to see your face, hair, neck and part of your shoulders. Don't take a close-up photo of the best parts of your face.
Shoot in color in most cases. Headshots are usually in color, but in the UK black and white photos are accepted. Whether your photo is in color or black and white, make sure the picture is well lit, so none of your features are hidden in shadows.
Get a photographer who knows how to take good pictures and communicate visually. You don't want a photographer who doesn't know how to communicate his client in a picture. That would have you looking stiff and boring. A good photographer will know how to pose you and light you in the right ways for an effective headshot.
Present your headshot with your recent resume that includes your contact information and basic stats (height, age, weight, eye and hair color, etc.). Your resume should be stapled to the back of the headshot facing outwards. This is so that casting agents can easily refer to it and remember who you are. You don't want your resume getting lost in the pile, separated from your contact information, and casting agents having no clue who you are.